a. Field of the Invention
The present invention relates to a curtain coater for coating a layer of liquid photographic coating composition on a continuous web in the manufacture of a photographic element.
Photographic coating compositions typically consist of aqueous solutions or dispersions of hydrophilic colloids with or without other materials dissolved or dispersed therein. They are liquid compositions of relatively low viscosity, for example, a viscosity of less than about 150 centipoises, and most frequently in the range from about 5 to about 100 centipoises, and after being coated onto the surface of the support are subjected to controlled temperatures to effect setting and drying. Coating compositions are in use in the photographic art which very much differ in chemical composition and also, to a more limited extent, in physical characteristics, and a number of different materials are in common use as the support. Thus, for example, the support may be made of paper, film base, glass, cloth and the like, and it may be coated in the form of discrete sheets or, as is more usually the case, in the form of a continuous web.
The manufacture of photographic elements is an extremely difficult art requiring extremely accurate control. Unlike coating operations in other arts, where complete coverage of the article being coated and attractive appearance are usually the only essentials for any particular coating method to find application in the photographic art it must provide for precise control. In particular, a photographic element requires individual layers which are extremely thin, i.e. a wet thickness which is maximum about 0.015 centimeters, and generally is far below this value and may be as low as about 0.001 centimeters. After coating the layers have to be set and/or dried before the product can be handled and their surfaces generally cannot be subjected to any physical treatment to increase their smoothness and/or their thickness uniformity. For this reason, the coating composition must be applied to the support in such a precise manner that after the layer is set and/or dried it will already be within permissible tolerances with respect to both thickness and uniformity. Since an individual layer must be extremely thin, as is indicated above, and since the maximum variation in thickness uniformity is mostly plus or minus a few percent, it will be appreciated that the coating operation in the manufacture of photographic elements is an unusually complex and demanding procedure. Moreover, the difficulties involved in meeting the requirements of extreme thinness and extreme uniformity are further magnified by the fact that in order to be commercially practical the coating operation must be capable of handling continuous webs with a width up to one meter or more and must permit the web to be coated at high speeds, for example, speeds as high as several hundred centimeters per second.
A particularly useful coater for realizing the aims set forth hereinbefore is a curtain coater. If such apparatus is arranged to provide exact control of the means by which the free-falling curtain is generated, and if certain critical relationships between the operating variables are maintained high quality photographic elements may be produced with this type of coater. Basic patents on the use of a curtain coater for the production of photographic elements are U.S. Pat. No. 3,632,374 relating to a single layer, and U.S. Pat. No. 3,508,947 relating to a multiple-layer curtain coater.
A phenomenon that may be noticed at coating speeds higher than approximately 150 m.min.sup.-1 is the displacement of the curtain in the direction of the web movement by the air entrained by the web. This displacement is moreover, not uniform since the curtain assumes a wavelike or undulating deformation, considered in the transverse direction of the curtain. The curtain deflections are largest at the web surface, and decrease to zero at the lip edge of the hopper. As a consequence of the curtain deformation, the coated layer gets longitudinal bandlike thickness deviations. These bandlike deformations are of the order of magnitude of only a few percent, and are not serious in the case of opaque photographic materials that are viewed or used in reflection. In the case, however, of photographic materials that are viewed in transmission, the density variations caused by bandlike thickness variations of one or more light-absorbing layers of the photographic material, whether these layers are light-sensitive or not, are unacceptable.
b. Description of the Related Art
It has been proposed to obviate the mentioned problem by means of different techniques.
First, it is known to provide the coater with shield means that extends parallel with the curtain and terminates closely to the web surface, with an end portion deflected in countercurrent direction. The shield means may occasionally be provided with a vacuum manifold operatively connected thereto for evacuating air from the surface of the web. Such an improvement is disclosed in U.S. Pat. No. 3,867,901. We have noticed that at speeds over 150 to 200 m.min.sup.-1, depending on the thickness of the applied layer, the mentioned shield means do not prevent the formation of bands in the coated layer. Further, the arrangement of the shield is critical since any contact of the lower edge of a shield with the web causes scratching of the web and the production of dust, and yet the distance should be almost zero in order to produce a satisfactory effect.
Another arrangement for the removal of the boundary layer of air from a web in a curtain coater is disclosed in FR-A No. 1,463,674. In this patent specification a coater is described in which a web is transported through a coating curtain by means of a conveyor roller before and after the curtain, and in which the web is slightly downwardly deflected by contact with a knife edge that forms an air-tight joint between the knife and the web, and is located a certain distance upstream of the curtain. According to an alternative embodiment of the arrangement, the knife is hollow and has an open edge at its underside, whereby the entrained air may be sucked off. The knife effectively removes the boundary layer of air from the web and also stabilizes the curtain, as well as the web, but its use is excluded in the manufacturing of photographic elements since the frictional contact with the support inevitably damages the surface of the support. Damaging of a delicate web will also occur by particles of dust and the like that become collected at the front side of the knife and that cause scratching of the web surface.
A still further arrangement for the removal of the layer of air that is entrained by the web, comprises a concave plate that is curved concentrically about the axis of the web-supporting roller and spaced therefrom over a distance not larger than about 1 mm. The narrow gap that is formed between the air shield and the web on the roller forms an important resistance to the air entrained with the web, and permits thereby the use of higher coating speeds. The mentioned arrangement is disclosed in Research Disclosure No. 18916 of January, 1980, but also with this arrangement a practical upper limit of the coating speed is formed by approximately 200 m.min.sup.-1 for a shield spacing of 1 mm. Smaller shield spacings can be used due to construction problems only for smaller curtain widths, such as curtain widths smaller than about 40 cm.
Finally, there is disclosed in DE-B-1,269,546 a curtain coater in which objects to be coated are transported by means of two endless belts through a coating zone. Disturbing influences of air displacements in the coating room and of air entrained by the objects are reduced by the provision of brushes that bear on the end of the straight advancing stretch of the first belt. The effect of the described measure is limited and is in fact advantageous only for the types of coating that are disclosed in this document, namely paints and adhesives.